Ionospheric long-term trends for South American mid-latitudes

Using ionosonde made observations at Concepcion (36.8 degrees S; 73.0 degre es W) for the 1958-1994 interval, long-term trends of critical frequency (f oF2) and peak height (hmF2) of the ionospheric F2-layer are analysed. The t rends found for different times-of-day and all seasons are consistent with an increasing diurnal-variation amplitude of both foF2 and hmF2. An increas ing hmF2 trend of up to 1.5 km/year found between midnight and dawn during winter has no precedent. It is suggested that these long-term amplitude cha nges may be associated with changes in the prevailing thermospheric meridio nal neutral winds. (C) 1999 Elsevier Science Ltd. All rights reserved.